The vulcanization of diene elastomers by sulphur is widely used in the rubber industry, in particular in the tire industry. In order to vulcanize diene elastomers, use is made of a relatively complex vulcanization system comprising, in addition to to sulphur, various vulcanization accelerators and also one or more vulcanization activators, most particularly derivatives of zinc such as zinc oxide (ZnO) alone or used with fatty acids.
Tire manufacturers seek solutions that allow zinc or its derivatives to be very greatly reduced or even, if possible, to be removed from their rubber formulations, due to the known environmental impact of these compounds, especially with respect to water and aquatic organisms (classified as R50 according to European Directive 671548/EC of 9 Dec. 1996).
It is found, however, that the reduction in the content of zinc oxide in particular in blends having a majority content of natural rubber such as those used in the tire treads of heavy-duty vehicles, leads to numerous drawbacks:
a drop in the number of sulphur bridges during vulcanization of the blends, which is expressed both by a reduction in the moduli of the blends and an increase in hysteresis, therefore an increase in the rolling resistance.
Of course, it would appear advantageous to remove or very greatly reduce the zinc by adapting the vulcanization system and, in particular, by increasing the sulphur content and also the content of other accelerators customarily used with zinc derivatives. However, a person skilled in the art knows that although increasing the content of sulphur and of accelerator actually makes it possible to attain a targeted bridge density, on the other hand it is at the expense of the compromise between the distribution of the sulphur network, that is to say the percentages of sulphur bridges formed with one sulphur atom, two sulphur atoms, more than two sulphur atoms, and the properties directly linked to the crosslinking such as the scorch time, the optimum curing time, etc. This being even more pronounced for elastomeric matrices predominantly based on natural rubber or synthetic isoprene.
In order to solve the problem of zinc elimination, it has therefore also been proposed to replace zinc oxide with another metal oxide, for example MgO, or else by a salt or oxide of a transition metal belonging to groups IIA, IVA, VA, VIA, VIIA or VIIIA of the Periodic Table of the Elements, particularly cobalt or nickel (see patent documents U.S. Pat. No. 6,506,827 and WO 2003/054081).
Such solutions are not really acceptable in the long term, from the point of view of environmental protection, in so far as they propose to replace one metal with another metal, while also doomed eventually to be dispersed in nature with the debris from the wearing down of tires, particularly that of treads that inevitably results from the various frictions due, for the most part, to the braking, acceleration and turning forces.